I've been using a 64gb SanDisk cruiser USB drive since I bought my tab. I know it is not a direct answer to your question, but the thumb drive works fine in the official USB adapter.

I can however see the whole drive in windows, both XP PRO, and Win 7. The drive came pre-formatted FAT32 from SanDisk, entirely one partition, and apparently readable fine on anything I have run across. From what I understand, you should be able to format your card NTFS if you are using pershoot's kernel.

One other thing to mention would be that there are tons of counterfeit SD cards on the market, and unless you got yours from a very reputable seller, there is a chance that it is hacked to report a higher capacity then it has.

The comment you made about being able to navigate the file system, but not open anything past 30g is pretty close to exactly how those counterfeit cards behave. I would be surprised if they used a 32g card as a base for the hacked card, but anything is possible... Anyhow, try H2testW, a google search will turn up download links. It is a free program that will write data to a target device until it fills the device. Then it will attempt to read it back and verify it's contents. After a certain number of mismatches, it will report the memory as likely defective.

This saved my butt a few times on some SDHC card purchases. It takes quite some time to run, especially with large drives, but the big plus here is that it verifies the flash memory before you put it in a device and loose a bunch of data that might be irreplaceable.

The card isnt a knock off, its from a reputable seller and the contents are available to browse ( incl. read write) on a normal desktop computer.... could it just be Android doesnt like 64gb SDXC cards?

I see, I thought you meant that after 30g you could just see the file structure but not access. I'm not sure if it is possible with the tab's USB otg adapter, but if you can format it with the tab, it might work. I know this has worked for people with phones that the manufacturer said was limited to 32g sdhc, phone would not read the full 64g until the people formatted with Android and card in phone.

Other than that, I'd try format ntfs (think only win 7 has this capability for flash) but there are stand alone apps, as well as Linux and parted... Other than that I don't have any other ideas... It might be it doesn't like the xc cards, but I don't think anything electrically changed with the card spec. This is complete speculation, but it seems that if it can handle the 64GB of nand flash in my thumb drive, it ought to be able to deal with the 64GB of nand flash in a sdxc card.

SDXC is different to SDHC in the same way that SDHC is different to SD.
Does the card reader indicate being able to support SDXC? If not, it's understandable why it is not working as expected and I'm a bit surprised it's partly working.
Are you using the same card reader for the computer with the Galaxy TAB?

At least with SD, they made a clear indication for different memory types / supported sizes. With XD, there was a 512MB limit where older devices would not recognise anything higher but the higher capacity cards were also just called XD.

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